Grey was fascinated by what he was seeing, he couldn't believe that there was a way an attack sequence could be made like this. It's not that he hasn't seen two people use their elements to complement each other's attacks, it's just that on such a large scale, he has never seen anything like this. The intricacies required to achieve something like this is unprecedented. For someone like him. It isn't difficult since he's the one who was using all elements which meant that he was using a single mind, these people on the other hand needs an insane amount of understanding of each other.
If Klaus and Reynolds were to attempt something like this, after a few tries, he's sure they will be able to get it. However, once Alice is added to the mix, the difficulty will increase exponentially.